The invention relates to rollers used in electrostatically assisted printing machines.
Printing machines have been developed in which the transfer of ink from a printing design cylinder to a web of non-conducting material, such as paper, is assisted by the electrostatic attraction of ink to the web. The electrostatic phenomenon allows for the transfer of ink with reduced mechanical pressure. The ink is transferred in a nip region that is formed where a resilient outer covering of an impression roller bears on the printing design cylinder through the web. In one example of the prior art, the nip region is defined by a flattening of the impression roller that extends about one-half inch along its circumference and further extends across the width of the web.
In printing machines of this type, the impression roller may be connected to one terminal of a voltage source and the printing design cylinder may be connected to the other terminal to apply a voltage across the web. Since the machine frame is typically grounded, the impression roller has been mounted in insulated bearings to prevent short circuiting a positive voltage applied to it. The voltage has been applied to a semiconductive covering which is formed around the insulated core of the impression roller. In the earliest machines a brush and slip-ring arrangement was used to conduct electrical current around the insulated bearings to this outer covering. In later machines the outer covering was charged by contact with a metal back-up roller that was pressed against the impression roller. As the impression roller is subject to more wear than the back-up roller, it was found advantageous to eliminate the brush and slip-ring arrangement which required service when the impression roller was changed.
In the packaging industry it is often necessary to run "partial webs" of a width shorter than the length of the impression roller. The operation of impression rollers on print cylinders in areas outside the web causes two problems. The first is sparking, which is due to the voltage difference across the relatively small or intermittent air gap between the impression roller and the print cylinder. The second problem is ink build-up in the areas outside the web.
One approach to solving these problems has been the selective application of voltage to the impression roller. This has been accomplished by removing the metal back-up roller and substituting a row of stainless steel sliding blades which are spaced along the length of the impression roller. Each sliding blade is movable between one position where it bears against the impression roller and another position where it is retracted, so as not to contact the impression roller. By selecting sliding blades which contact the impression roller in a middle region, voltage can be applied over the web, but not in areas outside the web. The sliding blades are not a suitable solution, however, because they are subject to unusual wear and also subject the impression roller to greater wear in the areas contacted by the blades.